[KafkaChannels] Pass-through "additional headers"

[travis-minke-sap]

The common MessageReceiver implementation forwards "additional headers" to the registered handler. These are non-ce headers that are deemed important to pass-through (x-b3, etc). Currently the KafkaChannel implementations do not forward these headers. Not forwarding such headers can, as an example, limit the ability to perform complete e2e tracing in Istio.

Proposed Changes

• 🎁 Enhance receiver to append "additional headers" into Sarama ProducerMessage.Headers
• 🎁 Enhance dispatcher to extract "additional headers" from Sarama ConsumerMessage.Headers
• 🎁 Enhance dispatcher to include "additional headers" in outbound requests.

Release Note

KafkaChannels will now pass-through the "x-request-id", "knative-*" and "x-b3-*" headers for all events.

[codecov]

Codecov Report

Merging #1001 (763eba4) into main (eaa0e79) will increase coverage by 0.04%.
The diff coverage is 85.00%.

@@            Coverage Diff             @@
##             main    #1001      +/-   ##
==========================================
+ Coverage   74.69%   74.74%   +0.04%     
==========================================
  Files         121      121              
  Lines        5675     5693      +18     
==========================================
+ Hits         4239     4255      +16     
- Misses       1213     1215       +2     
  Partials      223      223              

Impacted Files	Coverage Δ
...onsolidated/dispatcher/consumer_message_handler.go	0.00% <0.00%> (ø)
pkg/channel/consolidated/dispatcher/dispatcher.go	63.69% <0.00%> (-0.44%)	⬇️
...annel/distributed/dispatcher/dispatcher/handler.go	89.09% <100.00%> (+0.20%)	⬆️
.../channel/distributed/receiver/producer/producer.go	69.66% <100.00%> (+0.34%)	⬆️
pkg/common/tracing/tracing.go	100.00% <100.00%> (ø)

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-sandbox-eventing-kafka-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/channel/consolidated/dispatcher/dispatcher.go	66.1%	65.5%	-0.6
pkg/channel/distributed/dispatcher/dispatcher/handler.go	92.3%	92.5%	0.2
pkg/channel/distributed/receiver/producer/producer.go	78.1%	78.4%	0.3

[matzew]

/lgtm
/approve

thanks

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matzew, travis-minke-sap

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